Mark Levinson 390S reprogramming

Don't you have an owners manual? It sounds like you could have the wrong input selected or the wrong output selected. If you don't have a owners manual go here and download one:

http://marklevinson.com/downloads/index.asp

I got the following procedure from ML to reset the No. 390S:

1) Turn unit OFF (not standby).
2) Press and hold the front panel PAUSE button.
3) Continue holding PAUSE button while turning power ON.
4) The unit will display START UP, CODE XXXX, NEW DATA, DEFAULTS on the front panel display. Release the PAUSE button when the display stops changing.

Does anyone know where I can get the laser assembly for this unit?

Maybe here
http://www.encosystems.net/

Ah -

I would say that the Laser is failing. It should not be too difficult to find a replacement & tech to help w/ repair.
Keep us posted.